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这几天我有几个疑问，在dao层中可以完成对数据库的增删改查，那为什么还要在Service层中还要完成对数据库的增删改查呢？
我们先要了解一下：
mvc开发模式这样规定了，mapper和service属于不同的层次
首先看下面的代码
代码1：
//Dao层 操作数据库的 指定操作Student对象的增删改查基本操作 @Mapper public interface StudentDao
extends BaseMapper<Student> { }
代码2：
// 业务层快速开发 public interface StudentService extends IService<Student> { //
若封装好的增删改查不能完成我们所需要的操作，我们依然可以在下面进行方法书写，切记不要将方法覆盖 }
代码3：
@Service public class StudentServiceImpl extends ServiceImpl<StudentDao,
Student> implements StudentService { }
我们通过上面的代码可以发现
最上面的代码是属于Dao层
，完成对数据库的操作。Mapper针对的是Dao层的。在Mapper里面定义的是和数据库直接交互的（
一般来说是带有@Mapper注解的接口继承BaseMapper进行增删改查。）
下面的两端代码输入Service层，完成业务逻辑。Service战队的是业务逻辑层。Service里面是调用业务逻辑（
Service接口继承IService，实现类继承ServiceImpl。也可以使用Service类进行增删改查）。
首先我们要知道，我们使用的是MyBatis-Plus，Dao层和Service层中，已经完成了对数据库最基本的增删改查（代码1和代码2）且都是针对的Student类型的对象
